Whether working with stone, wood or other materials, good craftsmen strive for level (horizontal) floors and vertical walls in buildings being constructed by them. For well over a century, instruments known as spirit levels, carpenter's levels or simply levels have been used to help assure proper floor and wall construction.
Such levels commonly have an elongate frame in which are mounted two or more tube-like vials oriented at 90.degree. to one another. Such vials have an inner barrel-shaped cavity and are partially filled with a clear liquid which does not freeze at the lowest anticipated temperature. With the vial only partially filled, a bubble is visible in the vial and the positional relationship of such bubble with respect to reference lines on or around the vial is an indication of whether the frame is horizontal or vertical. An example of such a level is shown in U.S. Pat. No. 3,311,990 (Wright).
Typical level vials are made of hard material, e.g., glass or acrylic, and are fragile. Since levels are subjected to rough use around construction sites, vial breakage, abrasion and scratching are relatively common. Of course, breakage makes the level unusable while vial abrasion and scratching make it more difficult to see the position of the bubble.
Efforts have been made to protect level vials against breakage. One approach is shown in U.S. Pat. No. 2,755,561 (LaFosse) and involves an outer glass casing spaced from and surrounding a vial. The casing is resiliently mounted and because of the spacing between the casing and the vial, distortion of the appearance of the bubble seems at least possible.
Another approach to breakage prevention is shown in U.S. Pat. No. 1,384,062 (Happle) and involves a vial with relatively thick walls and a generally-square cross-section. Such vial is said to resist breakage and permit viewing the bubble from any point within a relatively wide viewing angle.
The level vials shown in U.S. Pat. Nos. 1,654,248 (Erdmann) and 3,368,287 (Ault) embody a different approach to easier vial and bubble viewing. The vials shown in such patents are illuminated, either by a phosphorescent material or by a lamp.
Yet another arrangement for protecting a level vial includes a transparent lens covering the opening in which the vial is mounted. Such lenses are made of polycarbonate or tempered glass, as examples. And such lenses must be mounted separately from the vial itself, an operation that has adverse implications for manufacturing costs and possible loss of the lens during use. Even with such vial protection, it is not unheard of for a user to break both the lens and the vial in a single incident.
Other patents show yet other arrangements for protecting a level vial. U.S. Pat. No. 3,871,109 (Vaida) shows a vial outer body having one or two insert members inside such body. In all versions of the Vaida device, the body is part of the liquid-containing structure, either as a "connector" holding the insert members together and/or as a wall in contact with liquid. The patent indicates that the body and insert members can be made of cellulose acetate butyrate or other materials but at least for some embodiments, the material selected must be sufficiently hard to be machinable. Cellulose acetate butyrate, presumably machinable, is used for such hard, non-resilient products as signal light lenses, signs, lawn sprinklers and tooth brush handles. Cellulose acetate butyrate has a hardness of about 30 to 115 on the Rockwell R scale.
The vial of the Vaida patent has bubble reference markings on the outward surface of the insert members or on the outside of the outer body. Separate opaque annular rings inside the vial are also mentioned as a way to provide reference markings.
U.S. Pat. No. 784,418 (Northoff) depicts a spirit level with an outer casing surrounding and in contact with a liquid-containing glass tube. The casing is apparently of opaque material, perhaps metal, since a viewing opening is provided in such casing. Such level is said to be durable.
While the prior art levels and level vials have been generally suitable for their intended purpose, they are not without disadvantages. Breakage, scratching and abrasion continue in spite of the efforts of earlier level and level vial designers.
Certain of the prior art level vials are simply too expensive to manufacture. The arrangement shown in the LaFosse patent would seemingly require significant hand labor for assembly. And to the extent that vial components depicted in the Vaida patent require machining, such manufacturing process would add enormously to cost.
An improved level vial overcoming some of the problems and shortcomings of the prior art would represent a significant advance in levels.